1. Field of the Invention
The present invention relates to a signal processing circuit for processing a detection signal from a bubble memory chip and more particularly, to a signal processing circuit for detecting a detection signal whose level "0" changes in accordance with changes in ambient temperature and in a rotating magnetic field.
2. Description of the Prior Art
In magnetic bubble memory devices, a rotating magnetic field 5 of 40 to 60 Oe, rotating in the major surface of the device, is applied to a propagation path so as to propagate bubbles having a diameter of 0.5 to 4 m. The propagation path has permalloy elements 1 of a configuration as shown in FIG. 1. In order for the bubble to stably exist, a DC bias magnetic field of 100 to 600 Oe must be applied in a direction perpendicular to a magnetic film surface. For storage of information, the presence of a bubble can represent information "1", while the absence thereof can represent information "0". A typical example of a magnetic bubble memory device of this type is described in IEEE TRANSACTIONS ON MAGNETICS, Vol 12, No. 6, pp. 614-617 (1976). In this memory device, a bubble detector comprises an expander 2 which includes a plurality of chevron propagation paths arranged in y direction and each having a plurality of chevron elements arranged in x direction, and sensing lines 3 and 4 each having a plurality of chevron elements, like the expander 2, which are electrically connected by permalloy bars in the x direction. The bubbles propagate along the propagation path in the y direction under the application of the rotating magnetic field 5 and are expanded by the expander 2 in the x direction. The expanded bubbles change the magnetized state of the sensing lines 3 and 4 when they pass under the sensing lines 3 and 4. More particularly, since the sensing lines 3 and 4 are made of permalloy, like the propagation path, these sensing lines 3 and 4 change their resistance owing to the magnetoresistance effect of permalloy. This change is detected as a voltage signal across each of the sensing lines 3 and 4 when constant currents are fed to the lines 3 and 4 from DC constant current sources 6 and 7 respectively connected to the sensing lines 3 and 4 as shown in FIG. 2.
The resistances of the sensing lines have noise components which change in accordance with the rotating magnetic field for driving the bubbles. The noise components change at a frequency which is twice the frequency of the rotating magnetic field. Lines 8 and 9 connected with the sensing lines 3 and 4 are affected by electrostatic and electromagnetic induction noise components generated from a coil for generating the rotating magnetic field. In order to eliminate these noise components, signals on the sensing lines 3 and 4 are supplied to a differential amplifier 10 through the lines 9 and 8. The differential amplifier 10 detects a difference between these signals and amplifies the difference by 10 to 100 times. An output signal from the differential amplifier 10 has a waveform 12 shown in FIG. 3a when a bubble is present. However, the output signal has a waveform 13 in FIG. 3a when no bubble is present. The output signal from the differential amplifier 10 is supplied to a comparator 11. The comparator 11 compares this signal with a threshold level (VTH) 15 in response to a read strobe (RSTB) 14.
The detection signal changes in accordance with changes in amplitude of the rotating magnetic field and ambient temperature. This signal also varies in accordance with irregularity of the chip manufacturing process conditions. Furthermore, the electrostatic and electromagnetic noise components from the coil for generating the rotating magnetic field differ from module to module. Therefore, the level of the output signal from the differential amplifier 10 changes as shown in FIGS. 3b and 3c wherein signals 17 and 19 correspond to level "0" signals, respectively, and signals 16 and 18 correspond to level "1" signals, respectively. These changes in signal level are also influenced by changes in temperature characteristics of the differential amplifier circuit and power supply voltage. In connection with FIGS. 3b and 3c, when the threshold level is fixed at a level 15 shown in FIG. 3a, erroneous operation occurs. In order to avoid this problem, the threshold level in FIG. 3b must be set at a level 20, and the threshold level in FIG. 3c must be set at a level 21.